Magnetic properties of titanium vanadium hydrides

Measurements have been made of the magnetic susceptibility X in ternary titanium-vanadium hydrides Ti_1?yV_yH_x in the range 0<y< 0.5 and with a hydrogen to metal ratio from 1.6 to 1.92; they cover the temperature interval from 80 to 440 K. The x^?T curves of some of the ternary hydrides possess a maximum similar to that seen in binary hydrides. Its occurrence depends upon both hydrogen and vanadium concentration. Comparison with the lattice constants shows that this dependence is related to the transition from the cubic γ-phase to the tetragonal δ-phase. The x^?T curves of those samples showing the phase transition were extrapolated from the cubic phase down to lower temperatures. Conclusions were drawn from the measured and extrapolated susceptibility values, as to the influence of both hydrogen and vanadium concentrations on the electronic structure of these hydrides. The dependence of the magnetic susceptibility on the vanadium concentration shows that, in the cubic phase, the fermi energy lies on the increasing side of a maximum in the density of states curve. The degree of occupation of the conduction band does not depend on the hydrogen concentration. The rigid band model cannot be used in the hydrides studied here to account for the effects of varying their hydrogen concentration.     

Dislocation interaction with hydrides in titanium containing a low hydrogen concentration

The dislocation configurations and their interactions with γ-hydrides were investigated in cyclically strained titanium containing 77?ppm hydrogen. At least three sets of dislocations could be activated in hydride precipitates. To accommodate the inhomogeneous strain between the matrix and the hydrides a crystal rotation, which is a rotation about the [001]_γ//[0001]_α axis, occurred in both of them. Accordingly cell structures with misorientations up to 10° about the same [0001]_α axis formed in the matrix near the interface. A mechanism for the crystal rotation and cell formation was proposed. Also hydride dissolution and strain-induced hydrides were observed at the intersections of slip bands with hydrides, and this was considered to be a process controlled by the hydrogen-transport effect of slipping dislocations.     

Sites and diffusion for muons and hydrogen in titanium hydrides

Low temperature sites for muons implanted in TiHx have been found to be a mixture of interstitial and substitutional sites, with substitutional occupancy determined by the probability that a muon in an interstitial site will have a vacant nearest neighbor substitutional site. As with ZrHx, activation from the interstitial site is observed below 300 K. From the depolarization rate in the substitutional site, the muon likely displaces the neighboring H atoms by about 0.1 A. Diffusion for the substitutional muons occurs above room temperature with an activation of about 0.38 eV, which is less than the 0.505 eV for hydrogen vacancy motion observed by NMR. To explain this the muon transition rate to a vacancy must be less than that of hydrogen.     

Electronic structure and equilibrium properties of hcp titanium and zirconium

The electronic structures of hexagonal-close-packed divalent titanium (3-d) and zirconium (4-d) transition metals are studied by using a non-local model potential method. From the present calculation of energy bands, Fermi energy, density of states and the electronic heat capacity of these two metals are determined and compared with the existing results in the literature.     

NMR studies of electronic structure and hydrogen diffusion in transition metal hydrides

Nuclear magnetic resonance spectroscopy has had extensive applications for the characterization of numerous metal-hydrogen systems. Although the greatest emphasis of proton NMR has been to evaluate diffusion behavior, increasing attention has been addressed upon the correlation of proton Knight shifts and the conduction electron contributions to proton spin-lattice relaxation times to the electronic structure properties of the hydride. The general principles of NMR, that pertain to the usual situations for most transition metal hydrides, will be briefly reviewed. Several specific examples from some recent research will be discussed in greater detail. In particular, the roles of host crystal structure and hydrogen site occupancy to hydrogen diffusion behavior are examined for the Ti_1-y Cu _y H _x and Zr_1-y Pd _y H _x systems. The proton hyperfine parameters in TiH _x and ZrH _x , as well as several related ternary hydrides, are used to qualitatively assess the character of the Fermi level electronic states. The relationship between the tetragonal distortions of the Ti and Zr dihydrides and a solid-state Jahn-Teller mechanism will also be examined. Mound is operated by the Monsanto Research Corporation for the U.S. Department of Energy under Contract No. DE-AC04-76-DP00053.     

Electronic structure and bonding in metal hydrides,studied with photoelectron spectroscopy

We present photoemission spectra of various binary and ternary metal hydrides and of some intermetallic compounds. An analysis of these data with respect to the known heats of hydrogen solution in the metals demonstrates two important properties of the metal-hydrogen bond: First we find that core level, shifts in ternary systems are not simply related to those in binary ones. In contrast to a frequently used assumption, metal-hydrogen interaction in a ternary hydride cannot be a pair interaction between the atomic constituents. Secondly, we find from our studies of the valence band spectra of some intermetallic compounds an inverse correlation between the heat of hydrogen solution and the density of states at the Fermi level.We analyse the core level shift data from binary hydrides using the experimental heats of hydrogen solution. We find a very good agreement between calculated and measured core level shifts in transition metal hydrides. However, in rare earth hydrides our approach fails. The reason for this behaviour originates in the photoemission process itself. A thermochemical interpretation of core level shifts can only be successful in the adiabatic limit of core excitation. The systematic behaviour of our results can be explained, if core excitation is considered to be adiabatic in transition metal hydrides but sudden in the rare earth hydrides. We also discuss the impact of such an interpretation on the concepts of adiabatic and sudden core excitation in metals.     

Electronic properties of hydrogen molybdenum bronze

The electronic properties of H_1.6MoO₃ were studied by different kinds of techniques. The static magnetic susceptibility is of the Pauli type above 210 K and of the Curie type below. UV-visible spectrometry permits an evaluation of the number of electrons given by the hydrogen to the oxide. The lifetime of the delocalized electron on molybdenum calculated by Fermi statistics is in agreement with the fact that the Mo⁵⁺ cation is observed by XPS and not by EPR. XPS measurements suggest that the conduction at room temperature is due to the interaction of the 4dt_2g orbital of the molybdenum with the p_π orbital of the oxygen. These results lead to the conclusion that H_1.6MoO₃ is, as far as the electronic properties are concerned, a bronze like Na_xWO₃ or Na_xMoO₃.     

Scaling properties of a tetragonal photonic crystal design having a large complete bandgap

Photonic crystal structures (PCs) of tetragonal lattice type are introduced and studied. They feature complete three-dimensional (3D) photonic bandgaps (PBGs). The PC design is based on two systems of ordered, parallel pores being perpendicular to each other. For increasing pore radii, the pore systems interpenetrate and an inverted woodpile geometry arises. The size of the 3D bandgaps depends on the ratio of the cell parameters L_x, L_y, and L_z, the pore radii and the refractive index of the dielectric material. If realized as a silicon/air structure, the maximum 3D gap is larger than 25%. A possible fabrication route for the near-infrared is based on 2D macroporous silicon where perpendicular pores are drilled, e.g., by focused-ion-beam etching. The dispersion behaviour of the PCs is theoretically analysed (band structures, density-of-states), systematically varying all relevant parameters. The optimization of the PBG sizes as well as a possible tunability of the PBG energies are discussed.     

Electronic structure of Ce and Sm in hydrides and the 4f collapse in YbHx

The population of the 4f, 5d, and 6s shells of rare-earth atoms in RH_x hydrides (R=Ce, Sm, Yb; x≈2–3) has been studied by the x-ray line-shift method. The population of the 5d and 6s shells of Ce and Sm atoms, and the charge on them in metals and hydrides, were determined from experiment and calculated within the Hartree-Fock-Dirac (Koopmans) model. The decrease of the charge on Ce and Sm revealed upon transition from the metal to the hydride argues unambiguously for the anionic (hydride) model. In YbH_x with x⩾2, the structural transition is accompanied by a strongly pronounced electronic transition from divalent to a noninteger-valence state . Fiz. Tverd. Tela (St. Petersburg) 40, 1393–1396 (August 1998)     

Electronic properties of a large quantum dot at a finite temperature

The physical properties of a two-dimensional parabolic quantum dot composed of large number of interacting electrons are numerically determined by the Thomas–Fermi (TF) method at a finite temperature. Analytical solutions are given for zero temperature for comparative purposes. The exact solution of the TF equation is obtained for the non-interacting system at finite temperatures. The effect of the number of particles and temperature on the properties are investigated both for interacting and non-interacting cases. The results indicate that the effect of e–e interaction on the density profile shows different temperature dependencies above and below a certain temperature T_c.     

Electronic structure of titanium monoxide with randomly distributed vacancies

The electronic structure of titanium monoxide TiO _y (0.810 ≤ y ≤ 1.262) in the high-temperature cubic phase with vacancies randomly distributed over the titanium and oxygen sublattices is calculated in the coherent potential approximation. The changes in the electronic spectra with the concentration of vacancies are retraced. The calculated spectra are compared to the available experimental data.     

Electronic and structural properties of hydrogen on semiconductor surfaces

In this paper we will review the scientific literature which addresses the atomic geometry and electronic structure of clean and hydrogenated semiconductor surfaces. In particular, results related to vibrational studies will be presented. First, surfaces of elemental semiconductors (Ge, Si), Ge/Si-alloys, and III–V compound semiconductors chemisorb in a first stage atomic hydrogen by saturating surface atom dangling bonds. In a second step surface bonds are broken and a change of the geometrical structure results. Finally, higher hydrogen exposures are able to etch semiconductor surfaces. Best understood to date are surfaces of Si(1 0 0), Si(1 1 1), Ge_xSi_1−x(1 0 0), and III–V's after cleavage which have been modeled by dimerized and undimerized structures. (1 0 0) surfaces of III–V semiconductors, like GaAs and InP, tend to be dimerized, too.     

Electronic structure and magnetic properties of YbCuAl

The electronic band structure of YbCuAl has been calculated using the self-consistent full potential nonorthogonal local orbital minimum basis scheme based on the density functional theory. We investigated the electronic structure with the spin–orbit interaction and on-site Coulomb potential for the Yb-derived 4f orbitals to obtain the correct ground state of YbCuAl. The exchange interaction between local f electrons and conduction electrons play an important role in the heavy fermion characters of them. The fully relativistic band structure scheme shows that spin–orbit coupling splits the 4f states into two manifolds, the 4f_7/2 and the 4f_5/2 multiplet.     

Electronic structure and properties of the actinides

We review the present state of understanding of the electronic structure and physical properties of actinide metals and intermetallic compounds as derived from relativistic APW energy band studies of some of the light (Th, U, Np, and Pu) and heavy (Am, Bk, and Cm) metals, the intermetallics URh₃ and UIr₃ and (NaCl structure compounds such as) UC. Emphasis is placed on the importance of Coulomb correlation and the role of actinide-actinide separation in determining the itinerancy (as in the light metals) or localization (as in the heavy metals) of the 5f electrons and in turn their resulting magnetic and other properties. The UX₃ systems considered are significant because the uranium sites are sufficiently separated to fall in the local moment f orbital range of the Hill superconductivity/magnetism plot but show no magnetic ordering. Comparisons to recent optical, de Haas-van Alphen and other data are given when available. The contrasting cases of the transition and rare-earth systems make it clear that the 5f electrons are a unique species which offer exciting challenges to both experimentalists and theorists.     

Magnetic properties of Co-ferrite-doped hydroxyapatite nanoparticles having a core/shell structure

The magnetic properties of Co-ferrite-doped hydroxyapatite (HAP) nanoparticles of composition Ca_10−3xFe_2xCo_x(PO₄)₆(OH)₂ (where x=0, 0.1, 0.2, 0.3, 0.4 and 0.5% mole) are studied. Transmission electron microscope micrograms show that the 90 nm size nanoparticles annealed at 1250 °C have a core/shell structure. Their electron diffraction patterns show that the shell is composed of the hydroxyapatite and the core is composed of the Co-ferrite, CoFe₂O₄. Electron spin resonance measurements indicate that the Co²⁺ ions are being substituted into the Ca(1) sites in HAP lattice. X-ray diffraction studies show the formation of impurity phases as higher amounts of the Fe³⁺/Co²⁺ ions which are substituted into the HAP host matrix. The presence of two sextets (one for the A-site Fe³⁺ and the other for the B-site Fe³⁺) in the Mössbauer spectrum for all the doped samples clearly indicates that the CoFe₂O₄.cores are in the ferromagnetic state. Evidence of the impurity phases is seen in the appearance of doublet patterns in the Mössbauer spectrums for the heavier-doped (x=0.4 and 0.5) specimens. The decrease in the saturation magnetizations and other magnetic properties of the nanoparticles at the higher doping levels is consistent with some of the Fe³⁺ and Co²⁺ which being used to form the CoO and Fe₂O₃ impurity phase seen in the XRD patterns.